1. Field of the Invention
The invention relates to a hand-held laser distance measuring device using a pulse reflection mixing method, in particular a hand-held construction laser distance measuring device.
2. Description of the Prior Art
In the building industry, distances must be exactly determined with an accuracy of within a few mm at a range of up to several hundreds of meters. The hand-held laser distance measuring devices which are suitably constructed for this purpose and to which the present invention is relates, use a pulse reflection mixing method of a modulated visible laser beam for measuring distances.
U.S. Pat. No. 6,917,415 discloses a hand-held laser distance measuring device in which a pulse reflection mixing method is used. In hand-held laser distance measuring devices of the type mentioned above in which a pulse reflection mixing method is used, commercially available laser diodes emitting in the visible red wavelength range, are used as laser sources. The emitted laser light is modulated by a series of very narrow spike pulses—hereinafter, transmitting pulse train—and bundled by a collimating lens to form a measurement laser beam. Accordingly, this special hand-held laser distance measuring device with pulse reflection mixing requires a series of very narrow laser pulses with a usual width of between 60 ps and 80 ps as transmitting pulse train. The pulse repetition frequency of the laser pulses ranging from 50 MHz to 200 MHz is very high compared to the pulse repetition frequency of several tens of kHz found in conventional hand-held pulse laser distance measuring devices, so that it is generally impossible to determine distances definitively at a range of up to several hundreds of meters distance with one measurement at a determined fixed pulse repetition frequency. Therefore, at least two measurements with two substantially different pulse repetition frequencies or differences of pulse repetition frequencies are needed for a definitive determination of distance, and even more to achieve a high accuracy for very large distance ranges. Using an algorithm, the control device determines the time differences—which are generally not definitive—between the reference pulses and the measurement pulses of the low-frequency mixing pulse train at different pulse repetition frequencies and, from the latter, determines the definitive distance from the rangefinder to the light spot on the measurement object with the help of the light velocity by means of a system of equations. The reference pulse train on the one hand and the measurement pulse train on the other hand, which are detected by the light detector, are directly subjected to direct mixing in the light detector followed by low-pass filtering. The direct mixing is controlled by a local oscillator pulse train which is locally generated at the measurement point and whose duty factor is equal to, or approximately equal to, the duty factor of the measurement pulse train and whose repetition frequencies are selected so as to differ slightly. Accordingly, the mixing pulse repetition frequency of the resulting low-frequency mixing pulse train corresponds to the amount of the difference between the pulse repetition frequency of the transmitting pulse train and measurement pulse train on the one hand and the pulse repetition frequency of the local oscillator pulse train on the other hand. This expands the time base by a greater factor (e.g., 1 million). Like the high-frequency detection pulse train (superposition of the measurement pulse train and the reference pulse train), the low-frequency mixing pulse train likewise comprises frequency-converted reference pulses and measurement pulses whose time delay is a measure of the distance. The low-frequency mixing pulse train with a low mixing pulse repetition frequency of, e.g., less that 1 kH, is low-pass-filtered, amplified, scanned by an analog-to-digital converter, and supplied to the control device which determines the time difference between the frequency-converted reference pulses and measurement pulses and, from the latter, the distance (which is possibly not yet definitive) to the measurement object which, as was described above, is definitively determined by multiple measurements at different pulse repetition frequencies. For further details relating to the generic hand-held laser distance measuring device with a pulse reflection mixing method, the person skilled in the art is referred to the above-cited document, whose disclosure is explicitly incorporated herein by reference thereto.
Since two slightly different pulse repetition frequencies are used in this method, the method is called heterodyne pulse reflection mixing. In heterodyne methods of this kind, all times of a pulse period are run through continuously, although the information content is found only within very short intervals, i.e., at the points of the reference pulses and measurement pulses. Since the reference pulses and measurement pulses make up only a very small percentage of the pulse period, only a small portion of the measuring time is made use of in the frequency mixing.